This invention relates to homogeneous charge compression ignition engines and more particularly to a method and means for controlling ignition timing and combustion rate in such engines.
Conventional homogeneous charge engines, as are extensively used in automotive vehicles, generally operate with spark ignition of a homogeneous charge of essentially stociometric air/fuel ratio with load control accomplished primarily by throttling of the charge, which reduces efficiency of the engine by creating additional pumping work. Conventional compression ignition, or diesel, engines on the other hand generally operate unthrottled and control the load or power output of the engine by varying the amount of fuel injected into the combustion chamber at the time at which ignition is desired to begin. While diesel engines have greater efficiency, they tend to emit higher concentrations of oxides of nitrogen (NOx) and particulate matter (PM) resulting from the combustion process.
A third method of combustion known as homogeneous charge compression ignition (HCCI) has some potential to significantly reduce NOx and PM emissions due to the homogeneous mixture of fuel and air providing a uniformly dispersed lean fuel mixture before the start of combustion near the end of each compression stroke of the engine. However, it has proved difficult to operate an engine in an HCCI mode over a wide speed and load range. Thus, at present, various combinations of conventional compression ignition combustion with HCCI combustion applied in different portions of an engine operating range are being considered for providing a fully operational engine operating method.
The present invention provides a method and apparatus for extending the practical range of operation of an engine with HCCI combustion, with the possibility of operating such an engine in the HCCI mode over a full operating load range and at least of extending the operating range of HCCI combustion in dual mode engine operation.
The method of the invention involves providing a controllable heater in the engine combustion chamber in a quiescent portion formed, for example, by a hot spot or recess in the chamber. The recess must be large enough and the heater surface and temperature great enough to provide for heating a segregated or quiescent portion of the charge to a proper temperature for ignition at a desired time of the engine cycle. The ignited charge must have sufficient mass to initiate flame combustion of the remainder of the charge through an open side of the hot spot or recess to obtain substantially complete combustion of the homogeneous charge. To operate the engine over a range of lean mixtures with unthrottled homogeneous charge CI combustion, the temperature of the heater and the temperature reached in the hot spot must be variable as necessary to obtain the proper ignition timing for burning the lean mixture in the cylinder over the full range of operating conditions of the engine and with load control provided by controlling the amount of fuel introduced into the unthrottled air charge of the cylinder.
In a preferred embodiment, the invention involves a combustion chamber hot spot recess, formed for example as a cone or other desired shape, having an open side to the combustion chamber and a relatively large surface within the recess on which a heater element is formed as a thin film or latticework of conductive resistance material. The heater element operates by resistance heating of the film when subjected to an electrical voltage. The conductive resistance heating material may be applied to an insulator which isolates the surface electrically from the combustion chamber walls. However, the heater support should also be partially thermally conductive to a cooled wall of the combustion chamber so that the element may be quickly cooled when the heat required is reduced or the heater is turned off completely. Thus, control of the temperature of the heater in the recess, which provides a variable temperature hot spot, may be effective to vary the energy applied to air fuel charges of varying degrees of leanness so as to provide the desired timing and combustion rate of the homogeneous charges supplied to the combustion chamber under the various operating conditions of the engine.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.